Research on the development of substitute energy has been actively conducted in order to cover the ever increasing amount of energy consumption. Commonly known power generation methods such as solar power generation, wind power generation, nuclear power generation, tidal power generation and geothermal power generation have been utilized, and in recent years, attempts have been made to utilize energy sources such as natural energy, bioenergy, and movement or heat of human bodies, as the future energy sources.
In the case of piezoelectric elements, studies have been directed to the applications in which piezoelectric elements are installed in the form of being inserted below the railways or roads so that the pressure exerted on the elements by trains and vehicles passing thereon is converted to energy and used. However, the recent instances of installing piezoelectric elements in subway stations in Japan, dance halls in Europe, and the like are intended to produce energy based on the pressure applied by people walking or running on such elements. It can be said that thermoelectric elements also belong to this category. A thermoelectric material is a material capable of converting heat energy to electric energy. However, while conventional power generation systems are systems for obtaining energy by operating turbines by burning fuels or by utilizing other forms of energy, thermoelectric elements have an advantage that heat can be directly converted to electricity without any additional intermediate processes. It can be said that one of the conditions necessary for electricity generation is heat energy, more specifically, temperature difference, and this implies that heat energy that is wasted in an unusable form in the course of power generation or use of other energy forms can be recycled.
Discovery of such phenomena was made in the early 1800s, and ever since, more specific physical understanding has been gained through extensive studies. Thus, quantitative evaluation criteria have been established for the characteristics of thermoelectric elements, and some of thermoelectric elements have been commercialized and are sold in the market.
The performance of a thermoelectric material, which is the thermoelectric figure of merit, is expressed by ZT=S2σ/k. Here, S represents the Seebeck coefficient; a represents electric conductivity; k represents thermal conductivity; and T represents the absolute temperature. The field of application of a thermoelectric material is determined by the extent of this ZT value. At the present, thermoelectric materials having ZT values that slightly exceed 1 are commercialized; however, this requires a temperature condition that is much higher than normal temperature. Thermoelectric elements that have been commercialized as such are based on inorganic semiconductor materials. Recently, active research on composites and nanostructures is ongoing in order to obtain improved thermoelectric characteristics.
Since inorganic semiconductor materials have high thermal stability and can be operated at relatively high temperatures, energy production can be attempted with the inorganic semiconductor materials by utilizing the waste heat produced by automobiles and industrial plants. However, since inorganic semiconductor materials are basically hard and crystalline, production of flexible elements is not feasible, and it is still difficult to solve problems concerning the toxicity of the materials themselves. Recently, studies have also been conducted to produce energy by utilizing not only high temperatures but also the heat produced by the human body. Numerous reports have been made from all over the world on examples of piezoelectric power generation, which is another case of enabling energy production through the energy of human bodies, and in which energy can be produced by utilizing the body weights of people. Use of thermal energy of human bodies requires yet a different approach. The scheme is not to use body weight without any contact of human body as in the case of piezoelectric power generation, but it requires maximal utilization of heat by bringing the material into contact with the human body. Also, it is required that such a material be capable of processing with high flexibility in conformity with the curvatures of the human body. The material should also be able to produce energy, not in an industrial environment that releases high temperature heat, but at a low temperature such as the body temperature. When these conditions are considered, it can be seen that existing inorganic semiconductor materials are not easily applicable due to their toxicity and hardness characteristics. In order to address these problems, research on organic thermoelectric materials is being actively conducted. Among others, studies on electroconductive polymers that can exhibit high thermoelectric performance have been conducted steadily.
Owing to the advantage that electroconductive polymers can substitute inorganic materials at low cost, attention has been paid to the polymers in connection with the applications for organic elements such as organic light emitting diodes, transistors, and organic photovoltaic cells. Since electroconductive polymers are basically organic materials, the structures can be easily modified compared to inorganic semiconductors, and changes in various physical properties can be induced. The most valuable advantage that can be obtained when an electroconductive polymer is utilized as a thermoelectric material is an increase in the ZT value induced by low thermal conductivity. This is because electroconductive polymers essentially have low thermal conductivity, and thus high thermoelectric figures of merit can be obtained. In addition, a thermoelectric composition based on organic materials basically has flexible characteristics, and therefore, the thermoelectric composition is highly advantageous in the production of elements, while element production can be achieved by simpler and inexpensive processes through printing methods. Based on these features, electroconductive polymers are used as thermoelectric composite materials, and are used to increase flexibility of elements. For instance, U.S. Pat. No. 5,472,519 suggests the possibility of using an electroconductive polymer as a thermoelectric material, and a method for producing an element from the polymer, and WO 2010/048066 and WO 2012/115933 suggest electroconductive polymers as supports for thermoelectric composite materials.
In addition to that, investigations have been conducted on the thermoelectric characteristics of nanoparticles and nanowires, and more attention is being paid to the potential of thermoelectric materials as organic nanoparticles and nanowires. For example, electroconductive organic nanowires have been reported in US Patent Application Publication No. 2004/0246650 and U.S. Pat. No. 7,097,757. An advantage of these nanoparticles and nanowires is that a high Seebeck coefficient can be obtained by regulating the state density of the constituent material.
When thermal conductivity is excluded from the thermoelectric figure of merit, efficiency of a thermoelectric composition can be determined through the output factor, which is an evaluation criterion for thermoelectric characteristics. The output factor can be expressed by a product of the square of the Seebeck coefficient and the electrical conductivity. Heretofore, despite their low thermal conductivities, organic thermoelectric compositions have not attracted much attention as thermoelectric materials due to low output factors. For example, Japanese Patent Application Laid-Open No. 2000-323758 describes that when a polyaniline is used as an electroconductive polymer, an enhancement of thermoelectric conversion performance is attempted through lamination, extension or the like; however, the polymer has low thermoelectric conversion performance and is not at a level suitable for practical use. Furthermore, U.S. Pat. No. 5,472,519 describes that poly(3-octylthiophene) as a conductive polymer and iron chloride as a dopant are used at a molar ratio of 2:1; however, the mixture has low electrical conductivity, and the thermoelectric conversion performance is not at a level suitable for practical use. In recent years, more attention is being paid again to electroconductive polymers as thermoelectric materials, owing to improved electrical conductivities attained through active research. Conductive polymers have an advantage that the electrical conductivity and the Seebeck constant values can be regulated by regulating the degree of doping, and thereby changing the carrier concentration. When the carrier concentration increases, increased mobility leads to an increase in electrical conductivity. On the contrary, when the carrier concentration decreases, the Seebeck coefficient is increased. In view of such relationships, reports have been made on studies conducted to obtain the maximum value for the maximum output factor, which is a product of the square of the Seebeck coefficient and the electrical conductivity by regulating the degree of doping (Olga Bubnova, et al., Nature Materials, Vol. 10 (2011), p. 429; and Re'da Badrou Aich, et al., Chem. Mater., Vol. 21 (2009), p. 751). Regarding the method for controlling the degree of doping, there are available a chemical reduction method and an electrochemical reduction method. The studies mentioned above respectively report the use of a chemical reduction method and an electrochemical reduction method. In the study conducted using a chemical reduction method, the output factor value was reported to be 324 μW/mK2 at the maximum. However, a chemical reduction method has limitations in precisely controlling the degree of doping of a conductive polymer. The second study in which the output factor is enhanced by an electrochemical reduction method does not provide excellent characteristics of the material itself; however, it is meaningful from the viewpoint that the degree of doping can be accurately controlled. However, there is a restriction that electrochemical reduction methods developed hitherto can be properly carried out only if a substance is formed on an electrically conductive electrode substrate. This implies that in order to actually produce a thermoelectric element by applying an electric reduction method, difficulties in the process associated with the use of an electrode substrate should be overcome somehow. If an electroconductive polymer itself has a sufficiently high electrical conductivity and can be utilized in electrodes, the problems mentioned above can be solved. In the present invention, a method of forming a highly electroconductive polymer film, and increasing the output factor by utilizing the polymer film itself as an electrode, will be explained. The inventors indeed confirmed that energy can be produced by this method through a difference between the skin temperature of the human body and the ambient temperature.